1. Field of the Invention
The present invention relates to a wet friction material used in a frictional engagement device such as a clutch and a brake used in oil in an automatic transmission and others of a vehicle such as an automobile.
2. Description of the Related Art
An automatic transmission of a vehicle, for example an automobile, normally includes a multiple disc clutch in which plural friction plates each of which is obtained by bonding a wet friction material to the surface of a metallic substrate (a core plate) and plural separator plates formed by a single plate such as a metallic plate as a subject material of friction are alternately arranged. A driving force is transmitted or disconnected by mutually pressing or releasing these plates in automatic transmission fluid (ATF) used for lubricating oil.
For such wet friction material used in the frictional engagement device used in oil, a paper wet friction material called a paper friction material is generally used. The wet friction material contains: a fiber base material such as a natural pulp fiber, an organic synthetic fiber and an inorganic fiber; a filler; a friction adjuster; and a binder, and a phenol resin is generally used as the binder, and diatomaceous earth is generally used as the filler. Such wet friction material is bonded to both surfaces of the friction plate in a wet multiple disc clutch. The wet multiple disc clutch is normally provided with four to five friction plates. Therefore, 8 to 10 wet friction materials equivalent to the double of them are used.
Recently, the miniaturization and the weight saving of automobile parts are promoted. The situation of the wet multiple disc clutch is similar, and by the reduction of the number of friction plates used in the wet multiple disc clutch, the reduction of an axial dimension (the miniaturization) and the weight saving of the wet multiple disc clutch are reviewed. From such a background, it is required that wet friction material with smaller number of friction plates undertakes the same torque as the above-mentioned conventional type wet friction material and has the same or a more superior frictional characteristic, heat resistance and durability, as compared with the conventional type.
Recently, a wet friction material having a high coefficient of friction is developed, and includes as a filler, a hard metallic powder such as alumina and silicon nitride. However, lately, a still higher coefficient of friction is required in a wet friction material.
It is generally known that the above-mentioned hard metallic powder has a problem that the powder attacks the frictional face of the subject material (that is, chips the face of the subject material such as the surface of a separator). In the meantime, while a granular diatomaceous earth used heretofore has no problem that it attacks the frictional face, it is difficult to acquire a high coefficient of friction.
Further, the wet friction material including alumina and silicon nitride as a filler to enhance the coefficient of friction of the wet friction material has a characteristic that the final coefficient of dynamic friction: xcexco is high, and thus there is a problem that when the wet friction material is used in a clutch, a shock at a gear shifting generates. A part of the wet friction material has a problem to absorb a moisture in air by the action of the hygroscopicity and the water absorption during storing products or transportation and generate the change in dimension.
The object of the invention is to provide a wet friction material having a high coefficient of friction.
Another object of the invention is to provide a wet friction material: having a high coefficient of friction; hardly damaging the mating frictional face; having an excellent heat resistance and durability; and further, having the small initial fluctuation of a coefficient of friction.
The other object of the invention is to provide a wet friction material: having a higher coefficient of friction, compared with conventional type wet friction material; showing a positive gradient in a xcexcs-V property [(the coefficient of static friction)-(the number of rotation)] so that the final coefficient of dynamic friction is kept low; and further, having an excellent dimensional stability.
(1) A wet friction material comprising:
a fiber base material;
a filler containing a disc-shaped diatomaceous earth; and
a binder (hereinafter also referred to as a xe2x80x9cfirst wet friction materialxe2x80x9d).
(2) The wet friction material according to the item (1), wherein the disc-shaped diatomaceous earth has an average diameter of 6 to 17 xcexcm.
(3) The wet friction material according to the item (1) or (2), which comprises the disc-shaped diatomaceous earth in an amount of 30 wt % to 50 wt %, based on total weight of the wet friction material.
(4) The wet friction material according to any one of the items (1) to (3), wherein the fiber base material contains cotton.
(5) A wet friction material comprising:
a fiber base material;
a filler containing at least one of a filler having Mohs hardness of 8 to 9.5 and a diatomaceous earth; and
a binder containing a silicone resin, the silicone resin being a hardened product of a hydrolyzed solution containing a silane coupling agent (hereinafter also referred to as a xe2x80x9csecond wet friction materialxe2x80x9d).
(6) the wet friction material according to the item (5), wherein the diatomaceous earth is a disc-shaped diatomaceous earth.
(7) The wet friction material according to the item (5) or (6), wherein the filler having Mohs hardness of 8 to 9.5 is alumina.
(8) The wet friction material according to any one of the items (5) to (7), wherein the silane coupling agent is represented by formula (1):
(R1)(R2)nSi(OR3)3xe2x88x92nxe2x80x83xe2x80x83(1) 
wherein R1 represents an alkyl amino group having primary amine at the terminal end; R2 and R3 each independently represents an alkyl group having 1 to 3 carbon atoms; and n represents an integer of 0 or 1.
(9) A wet friction material comprising:
a fiber base material;
a filler; and
a binder containing a silicone resin, the silicone resin being a hardened product of a hydrolyzed solution containing a mixture of a silane coupling agent represented by formula (1) below and a silane coupling agent represented by formula (2) below:
(R1)(R2)nSi(OR3)3xe2x88x92nxe2x80x83xe2x80x83(1) 
wherein R1 represents an alkyl amino group having primary amine at the end; R2 and R3 each independently represents an alkyl group having 1 to 3 carbon atoms; and n represents an integer of 0 or 1,
(R4)mSi(OR5)4xe2x88x92mxe2x80x83xe2x80x83(2) 
wherein R4 and R5 each independently represents an alkyl group having 1 to 3 carbon atoms; and m represents an integer of 1 or 2 (hereinafter also referred to as a xe2x80x9cthird wet friction materialxe2x80x9d).
(10) The wet friction material according to the item (9), wherein at least one of the silane coupling agents represented by formulae (1) and (2) has three hydrolyzable groups.
(11) The wet friction material according to the item (9) or (10), wherein a molar ratio of the silane coupling agent represented by formula (2) to that represented by formula (1) is 0.1 to 10.
(12) The wet friction material according to any one of the items (9) to (11), wherein the hydrolyzed solution contains water in an amount not smaller than the amount permitting hydrolyzing for half the number of hydrolysable groups contained in the silane coupling agents, but not larger than twice as much as the amount permitting hydrolyzing for all the number of hydrolysable groups contained in the silane coupling agents.
(13) The wet friction material according to any one of the items (9) to (12), wherein the filler contains at least one of a filler having Mohs hardness of 8 to 9.5 and a diatomaceous earth.
(14) The wet friction material according to the item (13), wherein the diatomaceous earth is a disc-shaped diatomaceous earth.
(15) The wet friction material according to the item (13), wherein the filler having Mohs hardness of 8 to 9.5 is alumina
The term xe2x80x9cwet friction materialxe2x80x9d used hereinafter includes all of the first wet friction material, the second wet friction material and the third wet friction material.
In the third wet friction material, the inventors paid their attention to a fact that a silane coupling agent was widely utilized for enhancing the performance of composite materials composed of an organic polymer and inorganic metallic material and earnestly reviewed the hydrolyzed solution of the silane coupling agent. A silane coupling agent is represented by formula: Yxe2x80x94SiX3, wherein Y represents a reactive organic functional group such as an amino group, an epoxy group, a vinyl group, a methacrylic group and a mercapt group; and X represents a hydrolyzable group such as an alkoxy group. For a mechanism of the action, a hydrolyzable group: X such as an alkoxy group reacts with water to generate a silanol group and it is bonded to a hydroxyl group on the surface of inorganic material. In the meantime, a reactive organic functional group; Y such as an amino group reacts with the reactive group in an organic polymer and is chemically bonded (is covalently bonded). That is, the silane coupling agent functions as an intervening agent between the inorganic material and the organic material and produces effects such as the enhancement of physical strength, the enhancement of the affinity of inorganics to the organic resin and the inhibition of the deterioration of physical strength under high temperature and high humidity. The binder is required to permeate uniformly the whole wet friction material when the silane coupling agent having such a characteristic applies to the paper wet friction material. Thus, the silane coupling agent is required to be particularly excellent in permeability and wettability to fine porous paper base material. From such a viewpoint, as a result of earnestly researching the hydrolyzed solution of a silane coupling agent, the inventors found that a silane coupling agent represented by formula: Yxe2x80x94SiX3, in which Y represents an amino group; and X represents an alkoxy group, was particularly excellent in permeability and wettability to the paper base material. Also, there is a slight problem in the dimensional stability of wet friction material because of the action of water absorption and hygroscopicity caused by the hydrophilicity of the hardened product of the hydrolyzed solution of the aminosilane. The simultaneous use of a silicon alkoxide having an alkyl group as means to solve the problem was effective and completed the third wet friction material of the invention.